Nickel-based superalloys are well-known in the art. They generally consist of a class of materials which solidify from the molten state according to monovariant eutectic reactions and provide aligned polyphase structures including such systems as the ternary and quaternary alloys identified as nickel-chromium-carbon and nickel-titanium-chromium-iron. Such compositions provide advantages and, in fact, are the subject of my copending application Ser. No. 347,677, filed May 5, 1989. As used in the present application, the term, superalloy, will be used to refer to high temperature alloys which melt at approximately 2500.degree. F. or more. Nickel, for example, melts at about 2500.degree. F.
Such nickel-based alloys are known to the prior art and are particularly useful, not only for their high melting points, but because after solidification, they may exhibit unusual strength. In particular, as disclosed in U.S. Pat. No. 4,111,723 to Lemkey et al., by means of directional solidification of a nickel-chromium-carbon alloy, chromium carbide fibers may be formed during the transition from the molten phase, which results in imparting great strength to the cooled alloy.
Niobium and its alloys exhibit properties that provide technological capabilities of great importance among the refractory metals. The advantages of niobium as compared with other refractory metals can be summarized as follows: the density, 8.57 gm/cc., and the thermal neutron absorption cross-section, 1.1 barns, of niobium are the lowest of the refractory metals. Its cryogenic ductility and ease of fabrication are excellent. Niobium oxidizes non-catastrophically; it is superior to both molybdenum and tungsten in this respect. It is in abundant supply; it is estimated that the accessible world reserves of niobium probably exceed those of molybdenum.
Although niobium is a ductile, soft metal at elevated temperatures, its strength can be improved by alloying to make it competitive with and superior to molybdenum and molybdenum alloys, its closest rival for use at temperatures in excess of 1500.degree. C. (2700 .degree. F.). The advantages of niobium alloys may well dictate their preferred use over other refractory metals in elevated temperature environments as high as 1850.degree. C. However, lack of oxidation resistance has been a major barrier to the use of niobium alloys in structural applications at high temperatures.
Initial studies of niobium alloys were directed to overcoming niobium's poor oxidation resistance. In common with other refractory metals, niobium and its alloys tend to oxidize in air at high temperatures, and this has seriously impaired its usefulness in elevated temperature applications. The chemical process is both complex and variable, involving repeated changes from linear to parabolic and vice versa, but although the chemical process is highly complicated, the transformation from the metal to the oxide state causes obvious thinning and concomitant weakening of the metallic structure.
Probably more significant than oxide formation is the high rate of diffusion of oxygen into the metallic structure, which produces regions of embrittlement. In point of fact, niobium can suffer as much as 70 percent loss of ductility with as little as 15 percent of its cross-sectional depth contaminated with oxygen. Indeed, that degree of contamination can be achieved in air within one minute at 1100 .degree. C. Alloying will improve niobium's resistance to oxidation weakening, but no elemental additives have been found which provide specific, enhanced protection against both effects.
As a result, while niobium has a melting point of about 3500.degree. F., 1000.degree. F. higher than the melting point of nickel, lack of oxidation resistance and less mechanical strength that might be desired have hindered the use of niobium as the base for a superalloy composition.
It is, therefore, a primary object of the present invention to provide a niobium-based superalloy composition in which the superalloy has enhanced mechanical properties over niobium, owing both to structural strengthening and to inhibition of oxygen weakening.